There is a growing international popularity for hollow blinds, due to their good thermal insulation, light transmittance and sun-shading properties. A so-called hollow blind mainly consists of a set of sun-shading sheets and a belt driven system which are sealed in hollow glasses. The driven system mainly includes both an upper belt gear and a lower belt gear. A belt is connected end to end by a drive handle and surrounds the two belt gears, wherein the upper belt gear is connected to a rope roller for the sun-shading sheets to control the sun-shading sheets to move up or down; and the belt must be in a tension state, so that the driving handle and the upper belt gear can make synchronized motions. This is usually achieved through precise adjustment of the length of the belt or precise adjustment of the distance between the upper and lower belt gears, which is a difficult problem for the hollow blind for the following reasons: a side frame is of a kind of profile with a rectangular cross-section, and a belt tensioning device along with the belt driven system are disposed in the interior of the profile, thus it is very difficult to adjust the tension of the belt. If the tension is too small, the belt will be easy to skip teeth, causing failure; and if the tension is too large, the resulting friction will be too large, making it difficult to move the blind up or down.